The invention relates to a device and a process for measuring current using the amplification of low value signals. It relates particularly to the field of automobile power steering.
Although the device usually used for automotive vehicle power steering uses a DC motor, it is envisaged to use an asynchronous triphase motor.
In power steering of an automotive vehicle, it is indispensable, to manage the control strategy, to know the torque applied by the power steering motor, and hence, in the case of a triphase motor, to know the electrical current passing through the three phases. The corresponding measurement is carried out by a shunt by using the conventional formula U=Rxc3x97I.
Moreover, when the vehicle driver turns the steering wheel slowly, a high demultiplication ratio between the shaft and the steering cannot be used because it would become very difficult for the driver to turn the steering wheel in case of failure of the system. It is thus necessary to use a motor with a high torque (in particular for xe2x80x9cheavyxe2x80x9d vehicles), even at low speed, and to apply to it a vectorial control which is actually the only one which permits a substantial torque at almost zero speed.
In practice, the electric motor used has a power which can be of the order of 500 watts, and it is supplied by continuous chopped voltage (the three phases are obtained by substantially rectangular chopping of the voltage from a vehicle battery, and smoothed by using the self effect of the motor itself). The frequency used is of the order of 15 to 25 KHz (namely substantially the top of the standard audio band).
To measure the voltage of the terminals of a shunt mounted in series on a phase of the electric motor (the shunt oscillating between ground and battery voltage, at cutout frequency), it is obviously desirable to reduce the heat loss by the Joule effect in the shunt (lost proportional to the square of the electrical strength passing through the shunt).
The strength being here of the order of 100 amperes, a shunt of 1 mÙ already has a power loss from heat of 10 watts. It will thus be understood that, because of this, the tendency is to seek to reduce further the value of the shunt.
The consequence of this choice of low shunt value is that the voltage finally measured at the terminals of the shunt is, for a shunt of 1 mÙ, of the order of 100 mV.
The problem is thus to carry out on the shunt a current measurement that is sufficiently precise, from a voltage whose order of magnitude is about 100 mV, and which acts on a chopped voltage from the 12 volt battery (the ratio is thus about 1% between the voltage to be measured and the chopped voltage), and chopped at a high frequency of the order of 15 to 25 KHz, in the presence of noise in the chopped voltage, particularly at each voltage shock (rising or falling voltage front).
The conventional differential amplifiers do not permit precise reading of the voltage at the terminals of the shunt, because the measurement is very much disturbed at each chopping transition. FIG. 1 shows the signal obtained by such a conventional differential amplifier for a value of maximum current.
The present invention meets the problem set forth above, and provides for this purpose, with the corresponding process, a device that is simple to make and low cost, permitting amplification and measurement of weak signals.
According to the invention, for measuring current in a line supplied by a voltage with noise and comprising a shunt mounted in series, there is used an amplifier of the signal of the shunt, hereafter called a floating amplifier, and means to supply said floating amplifier with a voltage which follows the supply voltage of the shunt.
It will be understood that the principle of the invention is to amplify the useful signal to facilitate its extraction from the chopped signal. To do this, the amplifier of the shunt signal is supplied with a voltage which follows the potential of the shunt.
The invention similarly provides for the application of the device and of the process which is its object, to a current measurement in a supply line of an asynchronous motor.
According to particular arrangements that may be used in combination:
the electric motor is of the triphase type,
the electric motor is supplied by a chopped voltage,
the electric motor has a power of about 500 watts,
the shunt has a value of about 1 mÙ,
the device comprises a differential amplifier whose inputs are connected on the one hand to the input of the shunt and, on the other hand, to the output of the so-called floating amplifier,
the floating amplifier has its inputs connected to the terminals of the shunt and is supplied from a mounting of the double bootstrap type.
These arrangements are favorable to a production of the device with conventional components, without particular requirements and hence of low cost, which makes the production of the device economical.
Finally, the invention provides, according to another aspect, the application of the device and of the process which is its object, to electrical power steering for an automotive vehicle.
Generally speaking, when there is involved a measurement of weak current in a line supplied by a voltage with noise, and comprising a shunt mounted in series, the process of the invention is characterized in that it comprises the step of signal amplification of the potential difference between the input and output terminals of the shunt by an amplifier supplied by a voltage which follows the supply voltage of the shunt.
Preferably, it moreover comprises a step of differential amplification of the difference between, on the one hand, the signal of the chopped voltage at the input of the shunt, and, on the other hand, the potential difference at the terminals of the shunt, amplified by the floating amplifier.
It is important to note that this problem of measuring weak current in the shunt in the presence of voltage noise is in fact new in an application of electrical power steering, to the extent to which most of the previously existing devices used DC motors, of lower power (100 to 150 watts). There thus was no particular problem to measure a voltage at the terminals of a shunt connected to the ground or to the battery voltage (because of the absence of chopping).
Present DC motors operate by using brushes, which give rise to a problem of wear, and which would not be usable in practice for motors of 500 watts. Moreover, the choice of having a constant torque no matter what the steering, and to reduce xe2x80x9ctorque oscillationsxe2x80x9d that now exist in power steering with DC motors with brushes, leads to using triphase asynchronous motors, and hence to introduce a chopped voltage, comprising a not inconsiderable voltage noise.
The invention is thus applicable more generally to all so-called xe2x80x9cbrushlessxe2x80x9d electrical motors, or to motors which do not operate with DC.
The description and drawings which follow permit better understanding the objects and advantages of the invention. It is clear that this description is given only by way of example, and not in a limiting way.